Pneumatic servo especially useful in extensometers



J. M. DENKER Dec. 13, 1966 2 Sheets-Sheet 1 Filed Oct. 19, 1964 T I I 1l 1 Dec. 13, 1966 J. M. DENKER 3,290,999

PNEUMATIC SERVO ESPECIALLY USEFUL IN EXTENSOMETERS Filed Oct. 19, 1964 2Sheets-Sheet 2 United States Patent chusetts Filed Oct. 19, 1964, Ser.No. 404,683 2 Claims. (Cl. 91-47) This invention relates to servos, andmore particularly to such devices especially useful in extensometers.

It is the object of this invention to provide a pneumatic servo capableof moving an element thereof in step with a specimen clamp grippingtransversely a specimen to be tested by longitudinal extension, andcausing this movement not only with a considerable force amplificationso that almost no longitudinal force is exerted on the specimen by theclamp, but at the same time with a very fast response and freedom fromoscillation or hunting, and with expenditures of very small quantitiesof fluid, all without any need to maintain the source of pressure at anyvery constant level.

Generally speaking, the invention features a piston mounted forlongitudinal reciprocal movement in a housing, the piston having each ofits two ends exposed to gas from a common source, the gas exertingpressure against different areas at the different piston ends and byvirtue of a restriction between the source and the piston end of greatereffective area being at a lower pressure thereagainst. In preferredembodiments said restriction is made adjustable for response control,and an adjustable valve for dumping control is placed between the sourceof pressure and the other end of the piston. In the most preferredembodiments, the larger piston end elfective area is divided into twoportions, one of which is separated from the other by a restrictioncausing a lag in the latters reaching the pressure over the former, togive especially effective stability.

Other objects, advantages, and features will appear from the followingdescription of the presently preferred embodiment of the invention,taken with the attached drawings, in which:

FIG. 1 is a vertical sectional view, partially broken away, of saidembodiment;

FIG. 2 is a side elevation of an extensometer incorporating two of saidembodiments; and

FIG. 3 is a plan view of said extensometer.

Referring now to the drawings, there is shown in FIG. 1 a cylinderindicated generally at (fabricated of elements, 12, 14, and :16) inwhich is seated for reciprocal movement piston 18. Mating cylinder andpiston surfaces are finished :to any accuracy such that no gaskets arerequired, and at the same time leakage therebetween is practicallyinsignificant, by well-known techniques. Housing caps 20 and 22 completethe housing.

Secured on the lower (as shown in FIG. 1) end of piston 18 is valveblock 24 in which is slide valve 26, downwardly biased by spring 28, andwith an adjustable downward stop screw 30.

Secured beneath valve block 24 is extensometer clamp arm assembly 3 2,which is pivotally mounted relative to valve block 24 through a flexibleleaf spring clamped between elements 34 and 36, and between element 38and base 40. Block 42 secured to base 40 includes blind hole 44cooperating with the head of screw 46 to limit travel of the clamp armassembly 32 about its axis of flexure. Teflon element 48 carrying spring50 yieldably urges specimen clamp base 52 against the mating seat ofelement 48.

Mounted on valve block 24 is LVDT (linear variable differentialtransformer) 56, leads to which extend up ice through (tgastightly) theservo. The core 58 of LVDT 56 engages screw 60 secured in bushing 6.2mounted on the valve block of another servo of the embodiment, thelatter being however upside down relative to the servo thus fardescribed, as indicated in partial dotted outline in FIG. 1, and shownin FIG. 2.

FIGS. 2 and 3 show my preferred embodiment of extensometer incorporatingmy above-described servos. Each servo has a pair of valve bodies 64 and66 which are seated respectively in bronze bushings 68 and 70 forrotation about a single vertical axis. The bushings 68 and 70 arecarried in servo supports 72 which extend from mounting base '74, whichcarries a pair of clamps 76 for mounting the entire extensometer on atesting machine. The specimen clamps 78 and '80 are also best shown inFIGS. 2 and 3.

Other elements of the preferred embodiment may most effectively bementioned in the following description of operation of each servo andthe extensometer.

Referring to FIGS. 1 and 2, air under pressure enters through conduit100, dividing then :to pass through conduits 102 and 104 tothe upper andlower supports 72. Within upper support 72, the stream divides again toflow upwards and downwards in conduit 106.

The air moving upwards through conduit 106 is adjustably valved by valve108 into conduit 110, through which it is in communication with pressurezone 112, which overlies half the effective upper end area of piston 18.The other half of the effective (to gas pressure, in terms of netdownward longitudinal forces) pressure on the upper end of the piston 18is exerted through cond-uit 114, adjustably valved by valve *116, inzone 117.

Air moving downwardly through conduit 106 passes through adjustablevalve 118 and conduit 120 into pressure zone 122, which has the sameeffective area as pressure zone 112.

The servo functions by maintaining the gap between spool valve 26 andexhaust hole 124 at such an amount as to keep the forces upward anddownward on the piston in balance, and since the piston upper end areais greater, the pressure in zones 112 and 1 17 must at equilibrium belower than that in zone 122. Thus, when specimen clamp 78 (or moves forexample downwardly, the spool valve 26 drops, decreasing its gap withthe exhaust hole 124. This increases the pressure above the piston,forcing the piston to follow the specimen clamp down until (when it hastracked it down the identical amount) equilibrium is againre-establis-hed. The servo operates on the same balancing way in thereverse direction, of course.

By using a greater effective area on one end of the piston, I makepossible operation with only a single exhaust gap, thus conserving gas.Furthermore, in conjunction with this approach, I have divided the uppereffective area between two pressure zones, with a restrictiontherebetween; while they come to the same pressure, this restrictioncauses a lag which acts as a sort of cushion, both increasing smoothnessof operation and very effectively combating oscillation or hunting.Stability is further aided by valve 118, which permits adjustably tuningdesirable damping here. Adjustable valve 108 permits varying the speedof response. I have provided thus a servo that will with great accuracytrack specimen clamp movement at 30 cycles per second or greater, at thesame time amplifying a force there in fractions of grams to one at thecylinder in hundreds of pounds.

Furthermore, since each piston is rotatable in its piston about avertical axis, while each cylinder is in turn rotatable about adifferent (but common to the cylinders) vertical axis, not only ismounting a specimen between the specimen clamps facilitated, but evenmore importantly undesired components of force on the specimen areavoided Specimen extension is, of course, the differential movement ofspecimen clamps 78 and 80, which in the manner indicated produces anelectrical signal through the LVDT.

I claim:

1. A pneumatic servo comprising a piston with a pair of oppositelydirected ends and a housing supporting said piston for reciprocalmovement, defining with said ends at least one pressure zone at each ofsaid ends, and including conduits from a common conduit separately toeach of at least one pressure zone .at each of said ends, one of theseparate conduits including a response restriction, and the totaleffective area of the piston end at least part of which is at the zonewith which the last-mentioned conduit communicates being greater thanthe total effective area of the other piston end, the piston end oflarger eifective area being divided into two pressure zones incommunication with each other through a stability restriction.

2. The servo of claim 1 in which said stability restriction isadjustable.

References Cited by the Examiner UNITED STATES PATENTS 1,512,804 10/1924 Roucka 91476 3,048,151 8/1962 Kudlaty 9l--4l7 FOREIGN PATENTS1,169,890 1/1959 France. 1,191,239 10/1959 France.

15 -MARTIN P. SCHWADRON, Primary Examiner.

EDGAR W. GEOGHEGAN, Examiner.

P. T. COBRIN, Assistant Examiner.

1. A PNEUMATIC SERVO COMPRISING A PISTON WITH A PAIR OF OPPOSITELYDIRECTED ENDS AND A HOUSING SUPPORTING SAID PISTON FOR RECIPROCALMOVEMENT, DEFINING WITH SAID ENDS AT LEAST ONE PRESSURE ZONE AT EACH OFSAID ENDS, AND INCLUDING CONDUITS FROM A COMMON CONDUIT SEPARATELY TOEACH OF AT LEAST ONE PRESSURE ZONE AT EACH OF SAID ENDS, ONE OF THESEPARATE CONDUITS INCLUDING A RESPONSIVE RESTRICTION, AND THE TOTALEFFECTIVE AREA OF THE PISTON END AT LEAST PART OF WHICH IS AT THE ZONEWITH WHICH THE LAST-MENTIONED CONDUIT COMMUNICATES BEING GREATER THANTHE TOTAL EFFECTIVE AREA OF THE OTHER PISTON END, THE PISTON END OFLARGER EFFECTIVE AREA BEING DIVIDED INTOW TWO PRESSURE ZONES INCOMMUNICATION WITH EACH OTHER THROUGH A STABILITY RESTRICTION.